Program for North American Mobility in Higher EducationIntroducing Process Integration for Environmental Control in Engineering Curricula Module 4: Environmental Challenges – Pulp & Paper Industry Caroline Gaudreault Created at: École Polytechnique de Montréal & Texas A&M University, 2003
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Tier II: Statement of Intent Tier II: Statement of Intent The purpose of this module is to demonstrate the application of the minimum manufacturing concepts using the Thunder Bay and Louisiana-Pacific case studies, and introduces the concepts of BAT and strategic planning. Tier II also includes some selected readings, to help the student acquire a deeper understanding of this subject.
Tier II: Content Tier II is broken into four sections: 2.1 The Thunder Bay case study 2.2 The Louisiana Pacific Samoa case study 2.3 Strategic long-term planning for kraft mills, technology roadmaps, and MIM metrics 2.4 Best Available Technologies for the kraft processes At the end of Tier II, there is a short multiple- answer quiz
2.1 REVIEW OF THE CLOSE-CYCLE OPERATING EXPERIENCE at GREAT LAKES FOREST PRODUCT LIMITED (Thunder Bay Mill)
A Little Bit of History In 1974, Great Lakes Forest Products Limited (GLFP) decided to build a second kraft line (kraft mill B) in their mill located in Thunder Bay, Ontario. As a result of this, the Ontario Ministry of the Environment (OMOE) asked them to construct a secondary treatment of the effluent in order to reduce BOD5, toxic elements and suspended solids discharged to the river. GLFP considered aerated lagoons but there was no close available space to accommodate it. This is why they began to discussed with Howard Rapson at the University of Toronto about the “Closed-Cycle Mill” concept. They endorsed the concept and submitted the project to the OMOE in place of secondary treatment.
A Little Bit of History (Cont’d) The OMOE gave the approval for the construction of the “close-cycle” kraft mill B, but with the provision than by March 1978, GLFP proves that: • The system to be operational; • To be as efficient as secondary treatment and, • Following this, GLFP propose to install the “close-cycle” system to the other kraft line (kraft mill A). The B mill began its operations in 1976 but the “close-cycle” operations were initiated only in 1977 after the salt recovery plant was completed. From 1977 to 1985, the “close-cycle” system was developed continuously in collaboration with the University of Toronto as well than with other players.
A Little Bit of History (Cont’d) Pitch deposits and scaling problems severely limited close-cycle operations, and more particulary during hardwood processing. In 1985, the close-cycle operations were discontinued for a lot of reasons including: • It was possible to obtain an equivalent BOD5 reduction only by increasing evaporator condensate use and recovery; • Operating costs were high. These costs include increased bleaching chemical costs and high energy costs for the salt recovery plant; • Heat exchangers in the salt recovery plant were corroded and it would have been very expensive to replace/upgrade them.
Information about Kraft Mill B • Bleached pulp production of 250 000 ton/year • First Canadian “effluent-free” pulp and paper mill • World’s first application of the Rapson-Reeve closed cycle concept
Bleach Plant Description • A blend of spruce and jackpine chips was fed Kamyr continuous digester. • The pulp was washed in a two-stage washer. • The bleach plant consist in 5 stages DCEDED: • DC: use of chlorine dioxide and elementary chlorine sequentially in the same stage, using ClO2 in large quantity prior to chlorine in the first stage will allow for an overall bleach chemical reduction, an increased yield and a preserved pulp resistance; • E: alkaline extraction (dissolution of reaction product with NaOH); • D: chlorine dioxide (reaction with ClO2 in acidic medium) • Whitewater is used to wash the pulp counter-currently.
Salt Recovery Process Description • Clarified white liquor from the recausticizing department was concentrated in an evaporator to increase the alkali content. • Precipitated sodium carbonate and burkeite (2Na2SO4.NACO3) were removed by clarification and cyclone separation system. • The clarified concentrated liquor was reconcentrated in an other evaporator to recover more 2Na2SO4.NACO3. • The recovered salt was treated with sodium hypochlorite to oxidize organic impurities. • Finally, after a sand filtration, the salt was used in the chlorine dioxide generators.
Key Features of the “Close-Cycle” System installed in Great Lakes Mill The main characteristics of the system were: • Dry drum debarkers; • Pressure (closed) primary knotters and screens; • Use of pulp dryer vacuum pump seal water in the wet end of the dryer and use of excess white water on the final D stage bleach plant washer; • Full countercurrent washing in the bleach plant; • A new salt removal process (SRP) based on evaporation of the white liquor in two stages to produce crystalline sodium chloride; • Use of excess filtrate from the E1 washer to dilute concentrated white liquor and to wash the unbleached pulp; • Use of excess DC filtrate after neutralization with caustic or white liquor for brown pulp washing and in the lime kiln scrubber, and subsequently for smelt dissolving; • 70% chlorine dioxide substitution for chlorine in the first stage to minimize chloride load to the recovery system; • An extensive spill collection and recovery system; • A stripping column to clean the foul condensates.
Overview of Water Recovery Water ClO2 Solution Bleach Plant DC Filtrate E1 Filtrate Brownstock Washers Digesters Evaporators Concentrated White Liquor Recovery Furnace Smelt Dissolver Causticizer White Liquor SRP Process Condensate Salt (Adapted from Springer & al., 2001)
Expected Benefits from the Close-Cycle Mill • Lower steam consumption • Greater steam production • Decrease of 1% fiber losses from screening, washing, etc. • Increase of 1% in bleached pulp mill • Lower consumption of bleach chemicals, and reduced salt cake and defoamer use • Elimination of external waste treatment • Reduction in kraft mill odor • Substantial annual operating cost savings
Corrosion: High temperature and chloride levels will increase the potential for corrosion in the close-cycle system. Even in bleaching plant equipment building material was carefully selected. Early evidence of corrosion was noted. Deposits: Hardwood runs were characterized by deposits from wood extractives, residues from defoamers and other sources which caused a lot of problems: pitch, scale, defoamer residues and talc/pitch deposits plugged washer fabrics and wires, washer nozzles and filtrate lines. Because of that, some filtrate recycle streams were discontinued during hardwood runs. Impact on Pulp Mill Operations The Bleach Plant Initial Design Problems: • A bad design of some of the bleaching tanks occasioned brightness problems and an increase in bleaching chemical consumption. • Air separation devices for the washers was not well conceived. This caused foaming problem during the washing stages. • The generation capacity of ClO2 was insufficient to meet the target of 70% substitution in the chlorination stage. The substitution averaged 50%. The capacity was increased. • There was no purge stream in the system. This resulted in an accumulation of Ca and so, in severe scaling problems. • Etc….
D C Closure: Prior to be recycled, the filtrate from this stage was neutralized with NaOH which result in a Ca/lignin/pitch precipitation that deposited on fabric and wire and in a impaired drainage. These deposit were removed using acid which result in corrosion. Also, because of the high organic content in the recycle stream, there was an increase in chemical consumption for this stage. E1 Closure: E1 filtrate was recycled to the brown decker which resulted in poor washing efficiency and carryover to the D C stage with incresed chemical consumption. This was corrected by recycling the E1 filtrate to washer where the solids content matched more the dissolved solids in the filtrate. Availability of the SRP, pluggage, pitch and scale had also severely hampered E1 closure. Impact on Pulp Mill Operations The Bleach Plant
Impact on Pulp Mill Operations Monitoring and Control • Maintaining water balances was difficult during startups, shutdowns, upsets and disturbances. • Because of they were upsetting the water balances, spill recovery was not very successful.
Impact on Pulp Mill Operations The Digester • No noticeable effect on the digester operations.
Impact on Pulp Mill Operations The Black Liquor Evaporators • Advanced corrosion in the black liquor evaporators reduced their life-time from 25 to 5 years. • The pulp mill operations were restricted due to a limited evaporator capacity. The later was increased but the spills were never effectively recclaimed. • 50% of total BOD5 of the kraft mill B was attributable to the evaporators condensate.
Impact on Pulp Mill Operations Recovery Boiler • Recovery boiler corrosion caused significant production losses and other costs.
Impact on Pulp Mill Operations Recausticizing and Lime Kilnb • Neutralized D C filtrate was used in the lime kiln scrubber. The filtrate ended up in the weak wash, green liquor, and white liquor. • The filtrate hampered dregs settling in the green liquor clarifier and lime mud settling in the white liquor clarifier. • A high dust loading reduced the kiln capacity by 10 to 15%. • The organic content of the filtrate had as consequence to form a yellow unreactive lime with variable slaking efficiency. Because of this higher temperatures were needed which result in higher fuel costs.
Corrosion: Salt-falls in the evaporators caused corrosion. Some pitting corrosion occurred on the vapor side of the crystallizer heat exchanger. Design: The SRP experienced poor steam econokmy and high energy costs. Low Availability: The original liquor solids removal system (thickeners and vacuum filters) were not effective in handling large crystal size distributions. It was replaced by a cyclone. Heater Fouling: Scale occurred in the evaporators that needed frequent washes. Impact on Pulp Mill Operations The Salt Recovery Plant
Obstacles to Increased Mill Closure • SRP reliability It was poor. After equipment and process modification the availability was adequate for 50% closure if the pulp mill uses 50% hardwood and 50% softwood. • Water balance problems The problems occurred during startups and shutdowns and there were problems recovering excess filtrates generated during upsets. • Equipment and process restrictions During the SRP shutdowns, there was no capacity to store and recover E1 filtrate used to dilute the SRP white liquor. • Pitch problems These occurred particularly with hardwood pulping and bleaching. • Recovery boiler corrosion Chloride input to the recovery cycle was restricted to reduce the potential for more corrosion.
Sucesses and Benefits • Change in the operating philosophy to one minimizing water inputs and controlling effluent discharges. • Low volume of effluent discharged from the bleach plant. • Some successful recycle of bleach plant filtrates to the chemical recovery. • Successful salt removal from the white liquor. • Use of removed salt in the chlorine dioxide generators. • High chlorine dioxide substitution pulp with not detectable levels of dioxines and furans. • Development of techniques to minimize impact of wood extractives. • During they operate in close-cycle, they assess secondary treatment for all their operations. They finally installed an oxygen activated sludge treatment instead of an aerated lagoon.
How Could Thunder Bay Have Beneficiated from PI • Process Simulation: Process simulation will have give more insight on the process and the potential build-up of contaminant. This way, interception equipment could have been put in place in order to avoid corrosion and other troubles due to contaminant build-up. • Mass Integration: The first advantage of mass integration is to target what is possible in terms of water reduction. Knowing the target helps in achieving the reduction. Furthermore, mass integration will have allow to implement the best trade-off between water reduction and cost. • Life Cycle Assessment: The main objective of the project was to avoid the secondary treatment. LCA could have help in evaluating the solution in term of environmental impact and convince the Ministry of Environment that a total closing of the water loop was not necessary.
References • DONNOVAN, D.A. A Review of the Closed-Cycle Operating Experience at Great Lakes Forest Products Limited 1977 – 1985. Tappi 94 Annual Meeting, Atlanta, US. • PATTYSON, G. RAE, R.G. REEVE, D.W. RAPSON, W.H. Bleaching in the Closed Cycle Mill at Great Lakes Forest Products Ltd.Pulp and Paper Canada, 82(6), 1991, p.212-220. • Great Lakes Paper Launches First Closed-Cycle Kraft Pulp Mill. Paper Trade Journal. March 15, 1977, p.29-34.
2.2 CLOSED-CYCLE TOTALLY CHLORINE FREE BLEACHED KRAFT PULP PRODUCTION at LOUISIANA PACIFIC’S SAMOA PULP MILL
Background • According to the US Clean Water Act, it was a national goal that the discharge of pollutants into the navigable waters be eliminated. • As a result of that, the kraft pulp industry had to develop pulping and bleaching technologies that can achieve this goal. • The most polluting wastewater was generated during the bleaching process. Chlorine, BOD, COD, color, toxicity, and dissolved organics was the major preoccupations. • Because they had exhausted many end-of-pipe pollution controls pulp and paper industry tried to innovate process changes that would improve wastewater quality from which the more promising were: • Find substitutes for elemental chlorine and hypochlorite bleaching agents • Reduce or eliminate bleach plant effluent
Background • Two bleached technologies had evolved to achieve the previous objectives: • Total Chlorine Free (TCF): Bleaching process that uses no chlorine compounds. • Elementary Chlorine Free (ECF): Bleaching process that uses chlorine dioxide. • These alternative bleaching technologies, more specifically TCF had made energy, process water and bleaching chemical recovery a possible approach to pollution prevention. • The European pulp and paper industry had first developed closed-cycle technologies, but in North America little progress has been achieved before Louisiana-Pacific's (L.-P.) project.
L.-P. Samoa Pulp Mill Plant Overview • Samoa bleached pulp mill is owned by L.-P. and located in a environmentally sensible area of the northern California coast. • The mill had a capacity of approximately 700 ton/day of bleached and unbleached pulp. • The next slide gives an overview of the process.
Samoa Mill Process Overview Fresh Water Fresh Water Fresh Water Chips Pulp Processing Bleaching Pulping Bleached Pulp Recycle Water Evaporators White Liquor Recovery Boiler Weak Wash Green Liquor Recausticizer Wastewater Discharge Fresh Water (Adapted from Louisiana-Pacific, 2000)
A Little Bit of History • In 1989, a lawsuit forced L.-P. to development a long-term plan for environmental improvement. Because of its location, a conventional secondary treatment was not considered appropriate by the EPA for the Samoa mill. Furthermore, building an secondary treatment was not an option for the Samoa mill because it was located in a costal dune habitat with endangered plant species. • In 1989, L.-P. implemented oxygen delignification and build a new recovery boiler. • In 1990, L.-P. proposed to eliminate the chlorine usage and to recycle part of the wastewater, but at this time only a few Scandinavian mills had implemented TCF (the environmental benefits were substantial but not well documented ). They removed all chlorine and chlorine dioxide facilities, added peroxide storage and distribution piping, and a stripper for condensates. • In 1994, Samoa became the first North American kraft mill to used completely TCF bleaching. They used peroxide and oxygen as alternative bleaching agents. • In 2000, Samoa was still the only North American mill to use TCF and successfully produced 5000 tons of TCF pulp
Positive Impacts of the TCF Process • The use of oxygen and peroxide facilitated the recycling of wastewater because they were less corrosive. • The use of oxygen delignification, the recovery of bleaching chemicals and the recycling of wastewater had the following benefits: • Cut bleach-plant effluent by 71% • Cut bleach-plant water usage by 50% • Cut mill process water usage by 31% • Reduced bleach-plant steam usage by 17% • Improved the wastewater clarity • Eliminated discharge of chlorinated organics into the ocean • Reduced plant odor
Negative Impacts of the TCF Process • Pulp brightness was reduced. However it proved with operator's experience of the TCF process. • The TCF process reduced the pulp production by 16%.
Close-Cycle Opportunities Offered by the TCF Process • L.-P. saw a need to increase TCF cost-efficiency, and a desire to cut wastewater discharge and continue reducing environmental impacts. • They also wanted to promote the TCF process in order to gain a competitive advantage over the competitors. • In 1995, they initiated a close cycle TCF (CC-TCF) with full bleach recycle (FBR).
Close-Cycle Upgrade:Project Objective • The goal was to commercialize the first strong, bright and cost-efficient kraft pulp mill in the world with a zero-effluent bleach plant • The first plant upgrades, which were completed in the late 1980s and early 1990s, allowed a reduction of about 71% of the effluent. The expectations were that the close-cycle plant modifications will allow for a nearly discharge free bleach plant.
Close-Cycle Upgrade:Project Approach • Adopting close-cycle posed the following problems: • Mineral buildup within the system; • Spills and overflow during start-up, normal operation, and shutdown; • Hydraulic control of internal process waste-water flows; • Control of transition metals that impact peroxide efficiency. • The project approach consisted in using existing mill data and computer simulations to evaluate alternative system configurations. Optimal alternatives were also tested using mill trials. The design process included new equipment, process synthesis, computer simulation, and trial-and-error to minimize capital cost. • The following technologies were added to enable to close-cycle operations: advanced green liquor filtration, extended digester cooking, and modified filtrate-recycle configuration.
Main Issues • The project took longer than expected because it was a trial-and-error process and the lack of demand for the TCF product. • A build-up of non-process elements such as potassium, chloride, magnesium and calcium necessitated the implementation of an advanced green liquor filtration system in order to increase the their purge capability. They also purged recovery boiler precipitator dust.
Close-Cycle Upgrade:Results and Benefits • Benefits of the CC-TCF process are summarized in the following table (these are additional to the ones due to the implementation of the other previous plant modifications): • The CC-TCF had some negative impacts on pulp quality (strength) but they were negligible.
Close-Cycle Upgrade:Environmental Benefits • Reduction in wastewater discharge and energy savings were not the only environmental benefits. The CC-TCF process also resulted in the following improvements: • Improved effluent color; • Lower BOD and COD loadings.
Close-Cycle Upgrade:Lessons • Staff dedication, persistence, and creativity are very important to the success of such a project. • Modifying plant processes is a more cost-effective approah to pollution prevention thant “end-of-the-pipe” wastewater treatment. • There is a lot to learn from others. L.-P. has inspired itself from the Scandinavian pulp plants which pioneered TCF pulp processes. • We must constantly re-evaluate the plant processes to seek improvement.
Continuous Improvement at L.-P. • After the CC-TCF project, L.-P. as identified an approach to further reduce wastewater by 38% in combination with heat recovery from the wastewater flows. Next step will be: • Oxygen Delignification Retrofit Conversion of the existing single-stage oxygen delignification system to a two-stage oxygen delignification unit in order to increase the degree of delignification achieved from the current 48% to 65%. • New Pulp Presses • Installation of 2 new pulp wash presses to improve the washing capacity (removal of most of the water from the pulp mat). • Recycling of the filtrate in previous wash stages. • This will allow for a lignin removal by the washing significantly greater .than that removed by the existing vacuum and improve the performance of the oxygen delignification stage and bleaching process.
Continuous Improvement at L.-P. Next step will be (cont’d): • Pressurized Peroxide Bleaching System Changing of the peroxide bleaching system for a pressurized one in order to reduce chemical consumption while improving pulp brightness. • Replacement of Bleached Pulp Cleaners Replacement of the current 35-year-old pulp cleaning system with one of a more modern design that will will be more efficient dirt removal from the pulp and may include additional processing capability to remove lighter particles such as plastics. • Focusing on solid wastes Usage of solid waste from green liquor filtration as a lime replacement in agricultural applications.
The EPA’s Cluster Rule • L.-P. experience with TCF process has been reviewed by the EPA and their research was integrated in the Cluster Rule’s development.
Characteristics of Candidate for Closed-Cycle TCF Process • Confronted with a necessary reduction and/or improvement in wastewater discharge and/or air emissions (for instance new regulation); • Confronted with an increase of water supply costs; • Confronted with an increase of wastewater treatment costs; • Lost of valuable by-products in wastewater streams; • Have completed some modernization.
How Could L.-P. Have Beneficiated from PI • Process Simulation & Mass Integration: L.-P. is already a success story. However, the do had some problems with NPEs and maybe their final water network is not optimal in terms of water/reduction and costs incurred. A combined process simulation and mass integration approach will have ensure them optimality or at least they will know they are not. • Life Cycle Assessment: The most advantages L.-P. can have come from LCA. They are opportunist and think in terms of competitive advantage. LCA will have enable to communicate their environmental friendliness.
References • Louisiana Pacific Corporation. Closed-Cycle Totally Chlorine Free Bleached Kraft Pulp Production at Louisiana Pacific’s Samoa Pulp Mill - Analysis of Business, Environmental, and Energy Issues, 2000, 54 pages.
2.3 STRATEGIC LONG-TERM PLANNING FOR KRAFT MILLS, TECHNOLOGY ROADMAPS, AND MIM METRICS
Strategic Planning Definition Long-term decision-making process by which an enterprise determines its strategic choices and the action programs aiming at the implementation of these choice. (Grand Dictionnaire Terminologique, 2004)